Norovirus (NV) is now recognized as the leading agent of diarrhea in patients of all ages worldwide and is known to be one of the most common causes of acute gastroenteritis. In developing countries, NV infection is estimated to cause up to 200,000 deaths in children under 5 years old. Diarrhea disease is the second leading cause of death for this age group. Recent studies have shown that NV is endemic in many communities, causing sporadic disease in addition to their classic outbreak clusters. Although the epidemiology in developed countries is well established, in developing countries this information is lacking due to the need to perform expensive molecular biology to detect the virus. In addition, knowledge of the effect of different genotypes and their clinical impact is lacking since there is no cell culture or simple animal model for this agent. Specific Aim 1. We will analyze mother-child and family-child transmission of NV by genotyping multiple region of the genome. We will conduct a nested case-control study of the children aged 0-2 years in Peruvian shanty town. Index case families will be compared with two age-matched control families by cluster analysis. We will determine the prevalence of family members infected with the same or different NV genotypic strains. Hypothesis 1. In the household of the index cases, the prevalence of other family member with NV infection is greater compared to that in the control households. Hypothesis 2. The chance that a child has the same strain of NV as the mother is greater than that with other household members. Hypothesis 3. Most of the adult family members with NV positive will be asymptomatic. Hypothesis 4. Children with diarrhea will have a genotype that is not circulating in the rest of the family. Specific Aim 2. We will evaluate whether specific variants of NV is a determinant factor of the severity of NV associated diarrhea or excretion time among children. Hypothesis 4. Variants will be associated with presence or severity of diarrhea. Hypothesis 5. Viral load and time of excretion of virus are associated with specific genogroup and its variants. This study will involve sequencing different regions of NV for typing, with the goal of determining whether there are differential contributions to the severiy of disease or transmissibility of the virus in the community. There are multiple primers available for sequencing NV, and we will apply these commonly used primers and also develop new primers to improve the reproducibility and grouping.